The cryptic wood white has been seen in Northern Ireland, but not in Great Britain. Photograph: Butterfly Conservation

A new species of butterfly previously unknown to science has been found flying in Northern Ireland.

The cryptic wood white looks exactly the same as both the Réals wood white and the wood white, a delicate and increasingly rare insect found in English woodlands in summer.

But scientists have discovered it is in fact a unique species that has far more chromosomes, and, genetically, it is 70,000 years old – far older than the two other species.

"It's a very exciting discovery. We are going to have to rewrite the butterfly books," said Martin Warren, chief executive of Butterfly Conservation.

The discovery comes as the charity launches the world's largest count of butterflies, the Big Butterfly Count, encouraging people to spend 15 minutes on a sunny day between 16 July and 31 July recording all the butterflies they spot in their garden, park or nearby countryside.

Last year 10,000 people in the UK downloaded free identification guides and submitted sightings of 189,000 butterflies online at www.bigbutterflycount.org.

Sir David Attenborough, the president of Butterfly Conservation, said butterflies rarely visited his suburban garden any more and the count was crucial to understand how and why they were in decline.

"I saw one peacock in my garden last year and that was a big day for me. I've even got to the stage where I welcome cabbage whites. They lift the heart but they are also crucial to the survival of British wildlife – for the birds that feed on their caterpillars and for pollinating flowers. If my heart is not going to be lifted by a butterfly because they've gone, my life is going to be much the poorer."

The count is being funded by Marks & Spencer with the retailer filling the breach left by an 85% cut in Butterfly Conservation's funding from Natural England, the government's conservation body, which is shedding 800 staff in the public sector cuts.

As well as providing crucial information on common butterflies suffering steep declines, including the small tortoiseshell and the meadow brown, the information submitted by butterfly lovers could help solve other mysteries in the butterfly world – and find more cryptic species, which are so called because they are identical to existing species and therefore hidden within them.

The cryptic wood white, which has the scientific name of Leptidea juvernica, was discovered by Spanish and Russian scientists who identified the new species across a swath of central Europe, including Germany, Romania and Northern Ireland.

Mysteriously, it does not occur in Great Britain, where the wood white is one of the most endangered species.

For the last 10 years in Northern Ireland the cryptic wood white was believed to be the Réal's wood white, another so-called cryptic species which was only discovered in Europe in 1988.

The cryptic wood white and Real's wood white differ from the wood white found in Britain in one important aspect: the males have much larger genitalia, leading to the nickname "long-willied wood white" in Northern Ireland.

Richard Fox of Butterfly Conservation said the discovery was important because it helped conservationists understand the distribution of rare insects and which species should be a conservation priority. "The fact that there have been two species hiding within the wood white is exciting," said Fox. "It also begs the question of what else is out there?"

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